Single Device for Gas and Flame Detection, Imaging and Measurement

ABSTRACT

A device images radiation from a scene. The scene can include two materials with spectral characteristics in different radiation wavelength regions. A static filtering arrangement includes two filters with different passbands corresponding to the two wavelength regions. An image forming optic forms an image of the scene on a detector. The radiation from the scene is imaged simultaneously through an f-number of less than 1.5 onto two detector pixel subsets. The imaged radiation on one pixel subset includes radiation in one wavelength region. The imaged radiation on the other pixel subset includes radiation in the other wavelength region. Electronic circuitry produces a pixel signal from each detector pixel. The pixel signals include information associated with absorption or emission of radiation in one of the respective wavelength regions by the two materials. The electronic circuitry determines the presence or absence of each of the two materials based on the pixel signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication No. 62/098,365, filed Dec. 31, 2014, whose disclosure isincorporated herein by reference. This application is related to theU.S. patent application entitled “Dual Spectral Imager with No MovingParts” (U.S. patent application Ser. No. 14/949,909), filed on Nov. 24,2015, the disclosure of which is incorporated by reference in itsentirety herein.

TECHNICAL FIELD

The present invention relates to the detection, imaging and measurementof infrared radiation.

BACKGROUND OF THE INVENTION

Industrial plants dealing with mining, production or storage ofexplosive or flammable gases and vapors such as hydrocarbons (methane,ethane, etc.), fuels of different kinds, hydrogen, acetylene, etc. arein constant danger of accidents. Explosions may cause fires, thus thereis inherent danger from both the explosion itself and from theconsequent ensuing fires. In addition, fires may result from a plethoraof diverse causes, and when occurring in such plants, such fires maythemselves cause explosions. The dangers are to both personnel andequipment, and the resulting damages may be in the worst cases loss ofhuman lives and large financial losses to the owners of the plants.

Additionally, the release of the gases in question has a negative impacton the environment. As a result, regulatory laws have been introducedaround the world to impose monitoring standards and heavy fines tocompanies that do not show due diligence in early detection of fires andprevention of inordinate releases of such materials.

The likelihood of explosions increases, up to a point, with increasinggas concentrations. Accordingly, over the past decades a large number ofgas concentration measuring devices and fire detection instrumentationhas been developed and used in mining, production and storage plants.Until recently only local detectors (for gases) or non-imaging IR and UVdetectors (for flames) have been deployed. A gas detector of this typecan easily miss the target gas if the gas cloud is present but does notphysically meet the position of the detector (or path in case of cloudmovement). This is due to the use of contact methods, such as chemicalreactions with the gas. In the case of fire detection, the monitor isbased on a single detector which does not provide an image of the field(i.e., scene) being monitored. Therefore the monitor cannot provide thenecessary information on the location and size of the fire.

Current industry instrumentation does not allow for the detection,identification, and location of the concentration, size and prognosisinformation of explosive gas or vapor clouds and flames due to incipientfires. Accordingly, current instrumentation cannot meet the additionalrequirements of being operable from a distance, in harsh environments,usually outdoors, and with minimal false alarms due to signals fromother possible infrared sources, such as sun reflections, welding arcs,halogen lamps etc. The alarms provided by such detection instruments maybe effectively used by the plant operators to prevent damages and lossesof human lives through a number of possible actions. An example of suchactions may be partial or total plant shut down, the request of firedepartment involvement, or other preventive or corrective action.

SUMMARY OF THE INVENTION

The present invention is a passive electro-optical instrument (i.e.,device), capable of detecting and imaging a cloud of hydrocarbon gasand/or a flame of burning material from a distance and distinguishingbetween the two types of materials.

The detection, imaging and measurement of hydrocarbon gas clouds andflames with the same device has a definite cost advantage over othermethods using dedicated infrared imagers for each of the two types ofevents. This solution requires fewer instruments, fewer installations,and less maintenance, and therefore reduced costs. Infrared radiationimaging and measurement technology combined in a single device is asuitable candidate for such an endeavor, since both hydrocarbon gasesand flames have spectral absorption and emission signatures in theappropriate range, as will be discussed in subsequent sections of thisdisclosure.

A key advantage of the device of the present disclosure, among otheradvantages, is that it provides the capability of event diagnosiswithout human intervention, so in addition to the above application itcan be used as a fixed installation for continuous monitoring and as ahand-held instrument for periodic plant maintenance and repair.

According to an embodiment of the teachings of the present inventionthere is provided, a device for imaging radiation from a scene theradiation including at least a separate first and second wavelengthregion, the scene including at least one of a first and second material,the first material having spectral characteristics in the firstwavelength region and the second material having spectralcharacteristics in the second wavelength region, the device comprising:(a) a detector of the radiation from the scene; (b) a static filteringarrangement including a first and second filter, each of the filtershaving a respective pass band and a stop band, the first wavelengthregion being within the pass band of the first filter and the stop bandof the second filter, and the second wavelength region being within thepass band of the second filter and the stop band of the first filter;(c) an image forming optical component for forming an image of the sceneon the detector, the radiation being imaged simultaneously, through anf-number of less than approximately 1.5, onto a first and second subsetof pixels of the detector, the imaged radiation on the first subset ofdetector pixels including radiation in the first wavelength region andthe imaged radiation on the second subset of detector pixels includingradiation in the second wavelength region; and (d) electronic circuitryelectronically coupled to the detector, the electronic circuitryconfigured to: (i) produce a pixel signal from each respective detectorpixel, each of the pixel signals including information associated withthe absorption or emission of radiation in one of the respectivewavelength regions by each of the first and second materials, and (ii)determine the presence or absence of the first and second materialsbased on the produced pixel signals.

Optionally, the first wavelength region includes radiation wavelengthsbetween 3.15 and 3.5 microns, and the second wavelength region includesradiation wavelengths between 4.3 and 4.6 microns.

Optionally, the detector includes a separate first and second detectorregion, the first detector region including the first subset of detectorpixels, and the second detector region including the second subset ofdetector pixels, and the device further comprises: (e) a radiationdirecting arrangement for directing radiation from a field of view ofthe scene through the image forming optical component onto the detector,such that the radiation is separately imaged onto the first and seconddetector regions through the f-number of less than approximately 1.5.

Optionally, the radiation directing arrangement includes a reflectivesurface positioned substantially parallel to the optical axis of thedevice.

Optionally, the first filter is disposed proximate to the first detectorregion and the second filter is disposed proximate to the seconddetector region.

Optionally, the first filter is a first plate interposed between thefirst detector region and the image forming optical component, and thesecond filter is a second plate interposed between the second detectorregion and the image forming optical component.

Optionally, the first and second wavelength regions are in the mid waveinfrared region of the electromagnetic spectrum and the detector issensitive to radiation in the first and second wavelength regions.

Optionally, each of the first and second filters is positioned at adistance, along the optical axis of the device, from the image formingoptical component, thereby allowing the radiation to be imaged throughthe f-number of less than approximately 1.5.

Optionally, each of the first and second wedge-shaped components ispositioned at a distance, along the optical axis of the device, from theimage forming optical component, thereby allowing the radiation to beimaged through the f-number of less than approximately 1.5.

Optionally, the radiation directing arrangement includes first andsecond substantially wedge-shaped components.

Optionally, the first filter is disposed on one of a first surface or asecond surface of the first wedge-shaped component, and the secondfilter is disposed on one of a first surface or a second surface of thesecond wedge-shaped component.

Optionally, the first surface of the first wedge-shaped component is aclosest surface of the first wedge-shaped component to the image formingoptical component, and the first surface of the second wedge-shapedcomponent is a closest surface of the second wedge-shaped component tothe image forming optical component, and the second surface of the firstwedge-shaped component is a closest surface of the first wedge-shapedcomponent to the scene, and the second surface of the secondwedge-shaped component is a closest surface of the second wedge-shapedcomponent to the scene.

Optionally, each of the first and second filters includes a plurality offilter elements, the plurality of filter elements being arranged suchthat each filter element of the plurality of filter elements of thefirst filter is adjacent to at least one respective filter element ofthe plurality of filter elements of the second filter.

Optionally, the determination of the presence or absence of the firstand second materials is based on the difference between the pixelsignals produced from the first and second subsets of pixels of thedetector.

Optionally, the first material is a hydrocarbon gas cloud and the secondmaterial is a flame.

Optionally, the electronic circuitry is further configured to: (iii) ifthe hydrocarbon gas cloud is present, provide a measurement of the pathconcentration distribution of the hydrocarbon gas cloud based on atleast a portion of the pixel signals.

Optionally, the first and second wavelength regions are in the long waveinfrared region of the electromagnetic spectrum, and the detector issensitive to radiation in the first and second wavelength regions.

Optionally, the static filtering arrangement is positioned at anintermediate focal plane, the intermediate focal being between the imageforming optical component and a second optical component.

There is also provided according to an embodiment of the teachings ofthe present invention, a device for imaging radiation from a scene, theradiation including at least a separate first and second wavelengthregion, the scene including at least one of a first and second material,the first material having spectral characteristics in the firstwavelength region and the second material having spectralcharacteristics in the second wavelength region, the device comprising:(a) a detector of the radiation from the scene; (b) a filteringarrangement including a first and second filter, each of the filtershaving a respective pass band and a stop band, the first wavelengthregion being within the pass band of the first filter and the stop bandof the second filter, and the second wavelength region being within thepass band of the second filter and the stop band of the first filter;(c) an image forming optical component for forming an image of the sceneon the detector through an f-number of less than approximately 1.5; (d)a mechanism for positioning the filtering arrangement relative to theimage forming optical component, such that, the radiation is alternatelyimaged through each of the first and second filters onto the samerespective pixels of the detector; and (e) electronic circuitryelectronically coupled to the detector, the electronic circuitryconfigured to: (i) produce, from each detector pixel, a respective pixelsignal for each alternation of the radiation imaged through the firstand second filters, the pixel signals including information associatedwith the absorption or emission of radiation in one of the respectivewavelength regions by each of the first and second materials, and (ii)determine the presence or absence of the first and second materialsbased on the produced pixel signals.

Optionally, the first wavelength region includes radiation wavelengthsbetween 3.15 and 3.5 microns, and the second wavelength region includesradiation wavelengths between 4.3 and 4.6 microns.

Optionally, each of the first and second filters includes a plurality offilter elements, the plurality of filter elements being arranged suchthat each filter element of the plurality of filter elements of thefirst filter is adjacent to at least one respective filter element ofthe plurality of filter elements of the second filter.

Optionally, the first material is a hydrocarbon gas cloud and the secondmaterial is a flame, and the electronic circuitry is further configuredto: (iii) if the hydrocarbon gas cloud is present, provide a measurementof the path concentration distribution of the hydrocarbon gas cloudbased on at least a portion of the pixel signals.

Optionally, the first and second wavelength regions are in the mid waveinfrared region of the electromagnetic spectrum, and the detector issensitive to radiation in the first and second wavelength regions.

Optionally, the determination of the presence or absence of the firstand second materials is based on, for each respective pixel of thescene, the averaging of a minority subset of pixel signals produced fromthe radiation imaged through the first filter, and the averaging of aminority subset of pixel signals produced from the radiation imagedthrough the second filter.

Optionally, the first and second wavelength regions are in the long waveinfrared region of the electromagnetic spectrum, and the detector issensitive to radiation in the first and second wavelength regions.

Optionally, the mechanism is configured to position the filteringarrangement at an intermediate focal plane, the intermediate focal beingbetween the image forming optical component and a second opticalcomponent.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a plot of the spectral absorptance of methane gas;

FIG. 2 is a plot of the spectral absorptance of ethane gas;

FIG. 3 is a plot of the spectral absorptance of propane gas;

FIG. 4 is a plot of the infrared emission spectra of flames of variousburning gas and liquid fuels;

FIG. 5 is a plot of the infrared emission spectra of cardboard and wood;

FIG. 6 is a plot of the frequency content of a fuel flame;

FIG. 7 is a plot of the self-emission spectrum of a hydrogen flame;

FIG. 8 is a schematic side view illustrating a device for detecting andimaging radiation from a scene in two separate wavelength regions,according to an embodiment of the invention;

FIG. 9A is a schematic side view illustrating a device for detecting andimaging radiation from a scene in two separate wavelength regions usinga checkerboard pattern filtering arrangement, according to an embodimentof the invention.

FIG. 9B is a schematic side view illustrating an alternate configurationof the device of FIG. 9A in which the detecting and imaging of radiationfrom the scene is accomplished with no moving parts;

FIG. 10 is a schematic representation of a checkerboard patternfiltering arrangement for performing detection and imaging of theradiation from the scene using the configurations of the device of FIGS.9A and 9B;

FIG. 11 is a schematic representation of an alternate configuration ofthe checkerboard pattern filtering arrangement of FIG. 10, according toan embodiment of the invention;

FIGS. 12 and 13 show schematic representations of groups of detectorpixels corresponding to a single scene pixel, according to an embodimentof the invention;

FIG. 14 is a schematic side view illustrating a device with a wedgeconfiguration for detecting and imaging radiation from a scene in twoseparate wavelength regions without moving parts, according to anembodiment of the invention;

FIG. 15 is a schematic side view illustrating the traversal of incidentrays from the scene and the scene background through the device of FIG.13;

FIG. 16 is a schematic front view illustrating a detector and theresulting image formed on the detector, according to an embodiment ofthe invention;

FIG. 17 is a schematic side view illustrating a device with a mirror fordetecting and imaging radiation from a scene in two separate wavelengthregions without moving parts, according to an embodiment of theinvention

FIGS. 18A and 18B are schematic side views illustrating filteringalternatives of the device of FIG. 14, according to embodiments of theinvention;

FIGS. 19A and 19B are schematic side views illustrating filteringalternatives of the devices of FIGS. 14 and 17, according to embodimentsof the invention;

FIG. 20 is a block diagram of image acquisition electronics coupled to adetector array, according to an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principles and operation of the device according to the presentinvention may be better understood with reference to the drawings andthe accompanying description.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not necessarily limited in itsapplication to the details of construction and the arrangement of thecomponents and/or methods set forth in the following description and/orillustrated in the drawings and/or the examples. The invention iscapable of other embodiments or of being practiced or carried out invarious ways.

The present invention is a device for detecting and imaging both a cloudof hydrocarbon gas and/or a flame of burning material. The deviceperforms the detection and imaging from a distance and can distinguishbetween the two types of events (i.e. hydrocarbon gas and flame ofburning material).

As examples, FIGS. 1-3 show the absorptance of 1 (ppm)×(meter) ofmethane (in units of Log₁₀ of inverse transmittance T), ethane andpropane in the 2800 to 3200 wavenumbers (cm⁻¹) range (equivalent to3.125 to 3.57 micron range).

Note that ethane and propane above and the other longer chainhydrocarbons butane, pentane and hexane, have absorptance between 3.3and 3.5 microns while methane absorbs in a wider range, from 3.15 to 3.5microns. Also note that none of such gases absorb infrared radiation inthe 4.3 to 4.6 micron range, where flames emit large amount ofradiation.

Typical emission spectra of flames due to various liquid fuels such asn-heptane, lead free, jet, diesel and others are shown in FIG. 4. Thefeature around 2.7 microns is due to self-emission of hot watermolecules in the flame, whereas the 4.3-4.7 micron feature is due to thehot CO₂ gas in the flame.

Similar infrared spectra of cardboard and wood are shown in FIG. 5. Thestrong features due to water (near 2.7 microns, due to water and 4.5microns, due to carbon dioxide) are similar to the flames of liquidfuels of FIG. 4.

Such flames also flicker with characteristic frequencies. Radiometricmeasurements of n-heptane flame and other fuel flames as a function oftime in both the 3 to 5 micron range and 8 to 14 micron range show that90% of the total emitted energy varies with frequency components up to5.5 Hz. With a fast enough camera gathering this information, theprobability of detection of a liquid fuel flame may be increased. FIG. 6shows the frequency content of n-Heptane flames as an example.

Note that in the 3 to 5 micron range the hydrogen flame emission is verysmall compared to the fuel flames emission. It is especially muchsmaller in the 4.3 to 4.6 micron range, where the flames due to liquidfuels show an especially large emission.

The absorptance data of the hydrocarbon gases are available to thepublic, for example, from Pacific Northwest National Laboratory inRichland Wash., USA, and are high resolution data. The flame emissionspectra and time/frequency behavior have been measured by an SR 5000 Nspectroradiometer of CI Systems, an instrument capable of measuringself-emission spectra of objects, calibrated in units of spectralradiance (Watts/((steradian)×(cm²)×40) or spectral radiant intensity inWatts/((steradian)×(μ)).

For the purpose of the present disclosure, it is useful to summarize thespectral data presented in FIGS. 1-7, as will described below.

Hydrocarbon gas absorption spectrum has a significant feature between3.15 and 3.5 microns in methane and between 3.3 and 3.5 microns in theothers. None of these gases have absorption in the 4.3 to 4.6 micronrange, where burning flames (except hydrogen) have a strongself-emission feature. The flames, represented by n-heptane in FIG. 6 asan example, show a time behavior of the infrared emission containingfrequency components up to 5.5 Hz.

The device of the present disclosure is applicable for use in industriallocations, and is of particular value for both in-door and out-door use,and to provide an alarm in a plant when an explosive gas may be found inabove than dangerous amounts, or when a fire has broken out in a spacewithin the field of view of the device. The device is preferably basedon an imaging camera (i.e., detector comprised of an array of detectorelements) sensitive in the 1 to 4.5 microns spectral range, where bothhydrocarbon gases and flames have strong spectral absorption or emissionfeatures. The 1 to 4.5 micron spectral range includes portions of theNear Infrared (NIR), Short-Wave Infrared (SWIR), and Mid-Wave Infrared(MWIR) regions of the electromagnetic spectrum. As will be furtherdiscussed, the detector array may also be sensitive to radiation in theLong-Wave Infrared (LWIR) region of the electromagnetic spectrum.Elements of the device include the optics to collect this IR radiationfrom the scene, a number of alternative spectral IR radiation filteringmethods, and suitable algorithms especially designed to extract theinformation needed for detection, real-time imaging and eventidentification from the resulting pixel signals.

1. General Elements of the Device of the Present Disclosure:

A camera (i.e., detector array) sensitive to infrared radiation in thespectral range preferably between 3 and 4.6 microns is built withcollection optics to receive such radiation from a scene and re-imagethe radiation on the camera through two band pass filters, one coveringthe range 3.15 and 3.5 microns and one covering the range 4.3 to 4.6microns. It is well known in the art that a gas cloud interposed betweena background and such a camera may be detected and imaged, and its pathconcentration measured (in units of (ppm_(volume))×(meter)), providedthe background temperature is different than the cloud temperature, andthe signals produced from the detector array are compared through aso-called in-band filter (transmitting radiation in the absorptionwavelength range of the gas, in our case 3.15 to 3.5 microns) and theso-called out-of-band filter (transmitting radiation outside theabsorption wavelength range of the gas): in this case the differencebetween the two signals is positive or negative depending on whether thetemperature difference between background and cloud is negative orpositive respectively. Analogously, from what is shown in FIG. 4 above,the filter transmitting 4.3 to 4.6 micron radiation is in-band withrespect to flames of burning fuels, methane, and solid materials; whilethe 3.15 to 3.5 filter is out-of-band with respect to the same flames(the signal will be higher in the former and smaller in the latterspectral range). In this way, if the camera pixels are exposed to bothfilters, either successively or simultaneously by using a split-imagemethod described below, the detection and identification of hydrocarbongases and flames can be achieved. The appropriate signal differencesthrough the two filters for each pixel will provide an indication as towhether the device is exposed to a flame (large and positive) or to ahydrocarbon gas (much smaller and positive or negative according to thebackground-object temperature difference).

The following features are important in this invention for becoming usedin practice, even though in principle are only optional.

The detector array used in the camera is preferably a PbSe uncooled orthermoelectrically cooled instead of other more expensive cryogenicallycooled detectors, such as InSb arrays, which are sensitive in the samespectral range. PbSe detectors are becoming available commerciallytoday. For example, St. Johns Optical Systems in Sanford and Lake Mary,Fla., US, offers such detectors, developed by Northrop Grumman, also inthe US. New Infrared Technologies (NIT), a company in Madrid, Spainoffers a number of PbSe array detector models.

Time or frequency analysis of the signals, in addition to thein-band-out-of-band comparison may be used in the mathematicalalgorithms of the device for better distinction between a gas cloud anda flame event, and between a flame and other infrared sources, yieldinglower false alarm rate. In fact, flames flicker at characteristicfrequencies that may aid in their identification.

Such PbSe detector arrays are sensitive to radiation in the MWIR regionof the electromagnetic spectrum. Alternatively, microbolometer typearrays may be used for sensitivity to radiation in the LWIR region ofthe electromagnetic spectrum.

2a. Gas Measurement:

In the following section, it is shown how the (ppm_(volume))×(meter) ofthe gas can be measured in a pixel successively exposed to the in-bandand out-of-band wavelength range by measuring the radiance difference inthese two ranges.

It has been well known for many years that it is possible to detect thepresence of a gas in the air by measuring the infrared self-emission ofthe background of the gas cloud in two different wavelengths, one whichis absorbed by the gas and one which is not, provided that thebackground and gas are not at the same temperature.

The radiance difference R reaching the measuring instrument between thetwo wavelengths w₀ (not absorbed) and w_(G) (absorbed by the gas), canbe expressed in terms of the background radiance B, the gas temperatureT_(G) (usually equal to the air temperature, and we assume that it isknown by measurement) and the gas transmittance t_(G) at the absorbedwavelength as follows:

R=B−B*t _(G)−(1−t _(G))*Pl(T _(G) ,w _(G))=(1−t _(G))*{B−Pl(T _(G) ,w_(G))}  (1)

where Pl(T_(G),w_(G)) is the Planck function at temperature T_(G) andwavelength w_(G). Two simplifications are used in equation (1) which arenot important for the sake of this explanation because the associatedphenomena can both be calibrated out in the more general case: i)atmospheric transmittance is assumed to be 1, and ii) backgroundradiance in and out of the gas absorption band are equal.

It is obvious from equation (1) that in the case that B is equal toPl(T_(G),w_(G)), the radiance difference R is equal to zero,irrespective of the value of t_(G), and in this case no information canbe inferred on the quantity t_(G). However, if B is different thanPl(T_(G),w_(G)), then equation (1) can be solved for t_(G) as follows:

$\begin{matrix}{t_{G} = {1 - \frac{R}{B - {{Pl}\left( {T_{G},w_{G}} \right)}}}} & (2)\end{matrix}$

All parameters on the right hand side of equation (2) are known: B isknown because it is measured in the non-absorbing wavelength w₀, Pl isknown because T_(G) is measured and w_(G) is known, and R is measured.Therefore t_(G) is known from equation (2). If the molecularabsorptance, A_(G), of the specific gas being monitored is known fromthe literature at w_(G), then t_(G) gives a measure of the product ofaverage gas volume concentration in the cloud, multiplied by thethickness of the cloud itself, or the so called concentration timeslength (or path concentration) value of the cloud. In fact, by theLambert-Beer law as follows:

t _(G) =e ^(−nA) ^(G) ^(l)  (3)

where l is the path length or thickness of the cloud and n is theaverage volume concentration of the gas being measured in the cloud,both corresponding to a specific pixel being examined. Equation (3) canthen be inverted to yield the value of the product nl for the particularpixel in question:

$\begin{matrix}{{nl} - {\frac{1}{A_{G}}{\ln \left( \frac{1}{t_{G}} \right)}}} & (4)\end{matrix}$

If t_(G) in (2) is measured to be less than 1, then nl in (4) is finiteand there is gas in the region of the pixel in question, in the amountnl (ppm_(volume))−(meter). If t_(G) from (2) is equal to 1, then nl=0 in(4), and there is no gas. Note that t_(G) values less than 0 or largerthan 1 are not physical, since t_(G) is a transmittance and is thereforebounded between 0 and 1.

2b. Flame Measurement:

In the case that a flame is present in a pixel of the scene instead of agas cloud, the detector pixel signal S_(flame) is nearly zero whenexposed to filter w_(G) and high when exposed to filter w₀. This is dueto the definition of the band pass of the two filters 4 a and 4 b and tothe shape of the flame emission spectra as shown in FIG. 4. Thedifference of the signals measured through the two filters 4 a and 4 b,or simply the w₀ signal (from the filter 4 b), indicates the presence orabsence of the flame in the corresponding scene pixel.

In the following sections, the various embodiments of a device will bepresented with different optical and filtering configurations forachieving the gas concentration measurement and flame detectionfunctionality previously discussed.

3a. Successive Exposure to in-Band and Out-of-Band Filtering:

FIG. 8 depicts an embodiment of a device 10-1 for detecting and imaginga cloud of hydrocarbon gas and a flame (i.e, a scene 80). The device10-1 includes an objective lens 2 (i.e., collection optics), positionedin front of a detector array 1 and a two-position filter holder or wheel3 containing two filters (a first filter 4 a and a second filter 4 b),either in front of the objective lens 2 or between the objective lens 2and the detector array 1. The first filter 4 a, centered at w_(G), isthe in-band gas filter with a pass band between 3.15 and 3.5 microns orbetween 3.3 and 3.5 microns, or an optimized range between 3.15 and 3.5microns. The second filter 4 b, centered at w₀, is the out-of-band gasfilter with a pass band between 4.3 and 4.6 microns. The first filter 4a (i.e., the filter centered at w_(G)) serves also as the out-of-bandflame filter; while the second filter 4 b (i.e., the filter centered atw₀) serves also as the in-band flame filter. The filter holder or wheelalternates the two filters in the optical train, successively exposingthe detector to the two different spectral ranges. Only the principalrays of the central, top and bottom pixels of the field of view (FOV)are shown. The filters 4 a and 4 b can be alternately placed between thelens 2 and the detector array 1 or between lenses in a multiple lenssystem design.

Note that the configuration of the device 10-1 as shown in FIG. 8 can bepreferably designed with a large numerical aperture of the objectivelens 2 to exploit the best possible detector sensitivity (or lowf-number, which is in general kept as close to 1 as possible, especiallywhen using uncooled infrared detector arrays). Accordingly, it ispreferred that the objective lens 2 of the device 10-1 has an f-numberless than 1.5, and as close to 1 as possible (i.e., f/1.5 or less). Adifferent configuration, using a dichroic beamsplitter to split theincoming beam into two beams to be filtered separately in the twowavelengths and two separate detectors can be used, but would be moreexpensive because of the additional detector cost. A further similarconfiguration using, besides the dichroic filter, an additional beamcombiner and chopper may be used to limit the design to the single arraydetector, but in this case the chopper, needed to switch between the twowavelengths in synchronization with the detector frame capture rate, isa low reliability moving part. These last two configurations requiremore complicated optics to avoid decreasing the numerical aperture ofthe focusing optics at the detector and degrade the device sensitivity.

Note that a whole frame image of the scene 80 is exposed to only one ofthe two band pass filters 4 a and 4 b in succession. The informationneeded for gas or flame detection and imaging is achieved by theacquisition of at least two frames while the two filters 4 a and 4 b aresuccessively positioned in the optical train by the rotation ortranslation of the holder or wheel 3, in such synchronization that eachframe is acquired through one of the filters (4 a or 4 b). The sequenceof exposure to the two filters 4 a and 4 b can be repeated as many timesas desired, whether for averaging to achieve higher signal to noiseratio or for any other reason. The sequence may be composed also byseveral frames through one of the filters and then several framesthrough the other filter, instead of alternating frames.

Image acquisition electronics 50 are electrically coupled to thedetector array 1 for processing output from the detector array 1 inorder to generate and record signals corresponding to the detectorelements (i.e., pixels) for imaging the scene 80. The image acquisitionelectronics 50 includes electronic circuitry that produces correspondingpixel signals for each pixel associated with a detector element. As aresult of the radiation being imaged on a multiple of detector elements,the image acquisition electronics 50 produces multiple correspondingpixel signals.

As shown in FIG. 20, the image acquisition electronics 50 preferablyincludes an analog to digital conversion module (ADC) 52 electricallycoupled to a processor 54. The processor 54 is coupled to a storagemedium 56, such as a memory or the like. The ADC 52 converts analogvoltage signals from the detector elements into digital signals. Theprocessor 54 is configured to perform computations and algorithms fordetermining and/or indicating the presence or absence of the gas cloudpath concentration distribution and/or flame, as well as imaging andmeasuring the gas cloud path concentration distribution and/or flame, asdescribed in Sections 2a and 2b, based on the digital signals receivedfrom the ADC 52.

The processor 54 can be any number of computer processors including, butnot limited to, a microprocessor, an ASIC, a DSP, a state machine, and amicrocontroller. Such processors include, or may be in communicationwith computer readable media, which stores program code or instructionsets that, when executed by the processor, cause the processor toperform actions. Types of computer readable media include, but are notlimited to, electronic, optical, magnetic, or other storage ortransmission devices capable of providing a processor with computerreadable instructions.

The above mentioned components of the device 10-1 are positioned withina casing defined by internal walls 30 of the device 10-1. Furthermore,the detector array 1 is preferably maintained within a detector case 12,which in turn is positioned within the casing of the device 10-1.

3b. Exposure to in-Band and Out-of-Band Filtering by Pattern Filtering:

FIGS. 9A and 9B depict different configurations of a device 10-2 and10-2′ which are alternative embodiments of the device 10-1, which usesan alternate method of switching the exposure of the pixels of thedetector to the two band pass filters centered at wavelengths w_(G) andw₀ by “patterned filtering”. This method may be implemented bothstatically (with some degree of loss of spatial resolution as explainedbelow) or dynamically by movement of an optical filtering device. In thelatter case the extent of movement is much smaller in amplitude than inthe method of Section 3a, and can be performed with a simpler andcheaper motor, like a piezoelectric oscillator, instead of a rotary ortranslatory motor like in the previous section.

Referring to FIG. 10, a checkerboard patterned filter 4 is implementedas the optical filtering device. The filter 4 can be used as areplacement for the two filters 4 a and 4 b of FIG. 8, and may be placedin an intermediate focal plane (FIG. 9A), an image plane of the scene,which is then re-imaged on the detector array, or on or as close aspossible to the detector plane itself (FIG. 9B). For the device 10-2,the positioning of the checkerboard patterned filter 4 in anintermediate focal plane is depicted schematically in FIG. 9A. The lenssystem of the device 10-2 includes the objective lens 2 and a re-imagingoptical lens 9 (i.e., re-imaging optics). For the device 10-2′, thepositioning of the checkerboard patterned filter 4 on the detector planeis depicted schematically in FIG. 9B. The size of the squares on theboard is optically matched to the size of the detector pixel, and eachsquare is coated so that a particular square corresponds to one or theother of the filters 4 a and 4 b.

In FIG. 10, the white squares (4 a-1, 4 a-2, . . . , 4 a-N) correspondto the first filter 4 a (i.e., the filter centered at w_(G)). Each whitesquare (4 a-1, 4 a-2, . . . , 4 a-N) represents an individual element ofthe first filter 4 a (i.e., the filter centered at w_(G)). Similarly,the diagonally hatched squares (4 b-1, 4 b-2, . . . , 4 b-N) correspondto the second filter 4 b (i.e., the filter centered at w₀). Eachdiagonally hatched square represents an individual element of the secondfilter 4 b (i.e., the filter centered at w₀). The elements of the firstand second filters 4 a and 4 b occupy the entire detector plane. Thefilter 4 is oscillated the length (i.e., amplitude) of a square alongthe horizontal or vertical direction relative to the detector plane.This successively exposes each detector pixel to one or the other of theband pass filters. The oscillation is performed in synchronization withthe detector frame acquisition (i.e., the image acquisition electronics50) in order to provide the necessary scene information for performingthe gas measurement and flame detection described in Sections 2a and 2babove, as carried out by the processor 56 of the image acquisitionelectronics 50.

The filter combination shown in FIG. 10 can be alternatively implementedwith alternating rows or columns of the elements centered at w_(G) andw₀ instead of a checkerboard, as in FIG. 11. In the implementation ofthe filter 4 depicted in FIG. 11, the movement of the filter 4 is in thevertical direction relative to the detector plane. The movementamplitude is equal to the length of one square, as previously described.

Note that many other similar configurations may be conceived, as forexample the alternating stripes of FIG. 11 arranged in columns insteadof rows. In this case the movement is in the horizontal directionrelative to the detector plane.

A checkerboard or stripe configuration as in FIGS. 10 and 11 may be alsostatic, either on an intermediate focal plane as mentioned above, orpositioned very close to the detector plane, so that each square or rowis exactly spatially registered with each pixel or row of pixels,respectively. As mentioned, such a configuration of the device 10-2′ isdepicted schematically in FIG. 9B. In this case the spatial resolutionor field of view is decreased because one scene pixel is now made of atleast two or four (or more) detector pixels. In order to obtain theinformation on the gas or flame presence, the signals of detector pixelscorresponding to either of the band pass filters is summed together andaveraged. The summing and averaging of the signals may be executed bythe processor 54. Accordingly, the entire scene 80 is imaged onto thedetector 1. The neighboring detector pixels produce signals, via theimage acquisition electronics 50, which correspond to the same portionof the scene 80 as filtered through each of the filters 4 a and 4 b.

In FIG. 12, a group of four detector pixels is shown which correspond toa single scene pixel. The signals of the pixels filtered through thewhite squares (i.e., 4 a-1 and 4 a-2 corresponding to the filtercentered at w_(G)) are averaged to obtain the in-band signal of thescene pixel. Similarly, the signals of the pixels filtered through thediagonally hatched squares (i.e., 4 b-1 and 4 b-2 corresponding to thefilter centered at w₀) are averaged to obtain the out-of-band signal ofthe scene pixel. In such a configuration, the number of scene pixels isreduced by a factor of two in both the vertical and horizontaldirections relative to the detector plane.

In FIG. 13, a group of two detector pixels is shown which correspond toa single scene pixel. In such a configuration, no averaging isnecessary, and the number of scene pixels is reduced by a factor of twoonly in the vertical direction relative to the detector plane.

As should be understood, both checkerboard patterned filterimplementations as depicted in FIGS. 10 and 11 can be used with of thedevices 10-2 and 10-2′, depicted in FIGS. 9A and 9B, respectively.Furthermore, the objective lens 2 of the devices 10-2 and 10-2′ can bedesigned with a large numerical aperture (i.e., f/1.5 or less and asclose as possible to f/1) similar to the objective lens of the device10-1.

Similar to the device 10-1, the components of the devices 10-2 and 10-2′are positioned within a casing defined by internal walls 30 of therespective devices 10-2 and 10-2′. Also similar to the device 10-1, thedetector array 1 of each of the respective devices 10-2 and 10-2′ arepreferably maintained within a detector case 12.

3c. Exposure to in-Band and Out-of-Band Filtering by Split Image WedgeConfiguration:

FIG. 14 shows an embodiment of a device 10-3 that uses an opticalconfiguration referred to as a “split image wedge” configuration. Theobject (scene 80 against the background 90) on the right side is imagedon the detector plane through the two wedge-shaped components (5 and 6)and the objective lens 2, so that two images of the scene 80 and thebackground 90 are formed on two halves of the surface of the detectorarray 1 (a first half 1 a and a second half 1 b), as shown in FIG. 16.The scene 80 and the background 90 are imaged simultaneously on twohalves of the detector plane, forming two identical images. The twoimages are formed through the two band pass filters centered at w_(G)and w₀, respectively, implemented as coatings 4 a and 4 b, respectively,so that each scene pixel is measured through an in-band and anout-of-band filter by two different detector pixels at the same time.

The wedge shaped components 5 and 6 together with the objective lens 2constitute collection optics 7. Most preferably, the detector array 1 ofthe device 10-3 is a PbSe type array sensitive to radiation in the MWIRregion of the electromagnetic spectrum.

The device 10-3 has the advantage that no filter movement is requiredand that the in-band and out-of-band signals are acquired at the sametime. This may improve on potential drifts between the two signals dueto gas cloud movement. The disadvantage is that the detector area isexploited for one half of the field of view that could be obtained withthe same objective optics and without the wedges. Similar to the device10-1 of FIG. 8 and the devices 10-2 and 10-2′ of FIGS. 9A and 9B,respectively, the device 10-3 includes image acquisition electronics 50for generating digital signals and for performing computations andalgorithms for determining and/or indicating the presence or absence ofthe gas cloud path concentration distribution and/or flame, as well asimaging and measuring the gas cloud path concentration distributionand/or flame.

The same infrared radiation from the scene 80 is imaged onto each of thetwo detector regions 1 a and 1 b, with each region of the detectorimaging the scene 80 in a different wavelength range.

FIG. 15 depicts the traversal of incident rays 42 a-42 f and 44 a-44 ffrom the scene 80 to the detector array 1. The broken lines between thescene 80 and the device signifies that the distance between the scene 80and the device as depicted in FIG. 15 is not to scale. In general, thedistance between the scene 80 and the device is much larger than thesize of the device itself, and is typically on the order of tens orhundreds of meters. Additionally, the broken line signifies that the twobundles of rays 42 a, 42 d and 44 a, 44 d both originate from the entirescene and not from one half of the scene.

Note that although only four incident rays 42 a, 42 d and 44 a, 44 d aredepicted in FIG. 15 (these are the marginal rays which define the fieldof view of the device 10-3 in the plane of the cross section defined bythe plane of the paper), it should be apparent that additional similarincident rays originating from the scene 80 are present and follow apath of traversal similar to the rays as described above. An exceptionis that ray components parallel to the plane of the page undergodeflection by the wedge, while the ones perpendicular to it do notundergo deflection. As such, reference to the incident rays 42 a, 42 dand 44 a, 44 d implicitly applies to all such similar incident raysoriginating from the scene 80 within the field of view.

The objective lens 2 focuses radiation deflected at an angle by thewedge-shaped components 5 and 6 on the detector array 1 to form twosimultaneous and separate images of the scene 80 with the background 90,each image being formed on one half of the detector surface. As such,the radiation from the scene 80 and its background 90 is imagedseparately and simultaneously onto the detector regions 1 a and 1 b.

The scene 80 and the background 90 is imaged by the device 10-3 with nomoving parts while maintaining a high numerical aperture and lowf-number (f/1.5 or less) at the detector array 1. This is accomplishedby positioning each of the first and second wedge-shaped components 5and 6 at a minimum fixed distance d from the objective lens 2 along theoptical axis of the device 10-3. Positioning the wedge-shaped components5 and 6 at a sufficiently large enough distance from the objective lens2, in combination with the above mentioned deflection angles, allows forthe low f-number (high numerical aperture) at the detector array 1 to bemaintained. This corresponds to high optical throughput of the device10-3. As a result, the same radiation from the scene is deflected by thewedge-shaped components 5 and 6 toward the objective lens 2 and imagedon the detector regions 1 a and 1 b through an f-number of thecollection optics 7 which can be maintained close to 1 (f/1) withouthaving to decrease the focal length f or increase the aperture diameterD. Accordingly, the minimum distance d which provides such high opticalthroughput can be approximately lower bounded by:

$\begin{matrix}{d > \frac{D}{2\; {\tan \left( \frac{\theta}{2} \right)}}} & (5)\end{matrix}$

where D is the aperture diameter of the objective lens and θ is thevertical field of view of the objective lens.

Having a large numerical aperture (low f-number) provides highersensitivity of the detector array 1 to the radiation from the scene 80,and less sensitivity to radiation originating from within the internalwalls of the device 10-3, the collection optics 7, and the opticalcomponents themselves.

As a result of positioning the wedge-shaped components 5 and 6 at thedistance d, the vertical fields of view of the wedge-shaped components 5and 6 are approximately half of the above mentioned vertical field ofview of the objective lens 2.

The wedge-shaped components 5 and 6 are preferably positionedsymmetrically about the optical axis, such that each is positioned atthe same distance d from the objective lens 2, and each is positioned atthe same angle relative to the optical axis. Such a design ensures thatthe same amount of radiation is imaged on the detector regions 1 a and 1b via the objective lens 2 from the wedge-shaped components 5 and 6.

As previously mentioned, the radiation from the scene 80 which is imagedonto the first detector region 1 a only includes one of the wavelengthranges. The radiation from the scene 80 which is imaged onto the seconddetector region 1 b only includes the other one of the wavelengthranges. This is accomplished by positioning the filters 4 a and 4 b inthe optical train.

In the exemplary implementation shown in FIGS. 14-16, the radiation fromthe scene 80 imaged on the first detector region 1 a only includes thein-band radiation from the gas filter 4 a (i.e., the filter centered atw_(G)), and the radiation from the scene 80 imaged on the seconddetector region 1 b only includes the in-band radiation from the flamefilter 4 b (i.e., the filter centered at w₀). Accordingly, the firstfilter 4 a filters radiation in spectral ranges outside of the firstwavelength range (i.e., stop band of the filter centered at w_(G)) andthe second filter 4 b filters radiation in spectral ranges outside ofthe second wavelength range (i.e., stop band of the filter centered atw₀). Thus, the radiation from the scene 80 that is directed by the firstwedge-shaped component 5 to be imaged on the first detector region 1 aincludes only the in-band radiation from the gas filter 4 a, and theradiation from the scene 80 that is directed by the second wedge-shapedcomponent 6 to be imaged on the second detector region 1 b includes onlythe in-band radiation from the gas filter 4 a.

As previously mentioned, the surface of the detector array 1 is dividedinto the two aforementioned regions by a dividing plane 8 as shown inFIG. 16. FIG. 14 includes a non-limiting exemplary representation of theCartesian coordinate system XYZ in which the detector plane is parallelto the YZ plane. Accordingly, the dividing plane 8 is parallel to the Zaxis and the optical axis is parallel to the X-axis. The wedge-shapedcomponents 5 and 6 are wedge-shaped in the XY plane.

In the embodiment of the device 10-3 shown in FIGS. 14 and 15, thefilters 4 a and 4 b are not necessarily optical elements from the opticsof the collection optics 7, but rather a coating on a first surface 5 aof the first wedge-shaped component 5 and a first surface 6 a of thesecond wedge-shaped component 6, respectively. The first surface 5 a isthe surface of the first wedge-shaped component 5 which is closest tothe objective lens 2. Likewise, the first surface 6 a is the surface ofthe second wedge-shaped components 6 which is closest to the objectivelens 2.

Additionally, a second surface 5 b of the first wedge-shaped component 5and a second surface 6 b of the second wedge-shaped component 6 may becoated with an antireflection coating. The second surfaces 5 b and 6 bare the respective surfaces of the wedge-shaped components 5 and 6 whichare closest to the scene 80. The antireflection coating providesincreased sensitivity of the device to the radiation from the scene 80.

Refer now to FIGS. 18A-18B and 19A, an alternative positioning of thefilters 4 a and 4 b. Similar to the embodiment of FIGS. 14 and 15, thefilters 4 a and 4 b are implemented as a coating, but in FIG. 18A thecoating is on the second surface 5 b of the first wedge-shaped component5. Similarly, in FIG. 18B, the coating is on the second surface 6 b ofthe second wedge-shaped component 6. In FIG. 19A the coating is on ornear the first and second detector regions 1 a and 1 b. Specifically,the first filter 4 a is implemented as a coating on or near the firstdetector region 1 a, and the second filter 4 b is implemented as acoating on or near the second detector region 1 b.

Refer now to FIG. 19B, an alternative implementation of the filters 4 aand 4 b. In FIG. 19B, the filters 4 a and 4 b are implemented asstationary plates positioned in front of, or in direct abutment with,the respective detector regions 1 a and 1 b.

In the filter alternatives illustrated in FIGS. 18A and 18B, the firstsurfaces 5 a and 6 a may be coated with an antireflection coating. Inthe filter alternatives illustrated in FIGS. 19A and 19B, the first andsecond surfaces of both wedge-shaped components 5 and 6 are preferablycoated with an antireflection coating. It is also noted that for clarityof illustration, the thickness of the coating and plates forimplementing the filters 4 a and 4 b is greatly exaggerated in FIGS. 14,15, 18A-1B and 19A-19B.

Similar to the devices 10-1, 10-2 and 10-2′, the components of thedevice 10-3 are positioned within a casing defined by internal walls 30of the device 10-3. Also similar to the devices 10-1, 10-2 and 10-2′,the detector array 1 is preferably maintained within a detector case 12.

3d. Exposure to in-Band and Out-of-Band Filtering by Split Image MirrorConfiguration:

A similar result of the device 10-3 may be obtained by using a mirror 9instead of the two wedge-shaped components 5 and 6 described in theprevious section (Section 3c). Such a device 10-4 is shown schematicallyin FIG. 17. In FIG. 17, the mirror 9 is positioned with respect to thecamera system (i.e., detector array 1) such that the reflective surfaceof the mirror 9 is perpendicular to the plane of the paper (XY plane)and parallel to the optical axis (X axis). Note that the same Cartesiancoordinate system XYZ used in FIG. 14 is also used in FIG. 17.

Although not shown in the drawings, the device 10-4 also includes imageacquisition electronics 50 similar to the embodiments of the devices10-1, 10-2 and 10-2′, and 10-3 for generating digital signals and forperforming computations and algorithms for determining and/or indicatingthe presence or absence of the gas cloud path concentration distributionand/or flame, as well as imaging and measuring the gas cloud pathconcentration distribution and/or flame. Furthermore, although not showin the drawings, the components of the device 10-4 are also positionedwithin a casing defined by internal walls of the device 10-4 and thedetector array 1 is preferably maintained within a detector case,similar to the devices 10-1, 10-2 and 10-2′, and 10-3.

FIG. 17 additionally depicts the traversal of incident rays 42 a-42 fand 44 a-44 d from the scene 80 to the detector array 1, similar to thedepiction of the traversal of rays shown in FIG. 15. The properties ofthe traversal of the rays depicted in FIG. 17 is generally similar tothe properties of the traversal of the rays depicted in FIG. 15 unlessexpressly stated otherwise and will be understood by analogy thereto.Furthermore, the definitions of the field of view of the device 10-4 andthe objective lens 2 of the device 10-4 are generally similar to thedefinitions provided with respect to the device 10-3 and will also beunderstood by analogy thereto.

The two filters 4 a and 4 b are placed either on planes where the twobeam bundles are separated (i.e., at the minimum distance d as afunction of the aperture diameter of the objective lens and the verticalfield of view of the objective lens disclosed in Section 3c and as shownin FIGS. 15 and 17), or directly covering each corresponding region ofthe detector, similar to the configuration depicted in FIGS. 19A and19B.

By accordingly positioning the filters 4 a and 4 b as mentioned above,the device 10-4 maintains a low f-number (f/1.5 or less) at the detectorarray 1, similar to that of the device 10-3.

Note that a specific property of the traversal of the rays depicted inFIG. 17 that is different from the traversal of the rays depicted inFIG. 15 is lack of the additional reflected rays which pass through thesecond filter 4 b. Specifically, only the first bundle of rays (42 a and42 d) is reflected by the mirror 9 before passing through the firstfilter 4 a and the objective lens 2, whereas the second bundle of rays(44 a and 44 c) is not reflected at all and passes directly through thesecond filter 4 b and the objective lens 2. In other words, the mirror 9inverts the rays traversing the filter 4 a in a vertical upside downdirection with respect to the ones of 4 b and as a result the two imagesof the scene 80 formed on the detector array 1 are upside down withrespect to each other.

It will be appreciated that the above descriptions are intended only toserve as examples, and that many other embodiments are possible withinthe scope of the present invention as defined in the appended claims.

What is claimed is:
 1. A device for imaging radiation from a scene theradiation including at least a separate first and second wavelengthregion, the scene including at least one of a first and second material,the first material having spectral characteristics in the firstwavelength region and the second material having spectralcharacteristics in the second wavelength region, the device comprising:(a) a detector of the radiation from the scene, the detector having adetector surface and including: (i) a first region of the detectorsurface including a first subset of detector pixels, and (ii) a secondregion of the detector surface, separate from the first region,including a second subset of detector pixels; (b) a static filteringarrangement including a first and second filter, each of the filtershaving a respective pass band and a stop band, the first wavelengthregion being within the pass band of the first filter and the stop bandof the second filter, and the second wavelength region being within thepass band of the second filter and the stop band of the first filter;(c) an image forming optical component for forming an image of the samescene on the detector, the radiation being imaged separately andsimultaneously onto the first and second regions of the detectorsurface, the imaged radiation on the first detector surface includingradiation in the first wavelength region and the imaged radiation on thesecond detector surface including radiation in the second wavelengthregion; (d) electronic circuitry electronically coupled to the detector,the electronic circuitry configured to: (i) produce a pixel signal fromeach respective detector pixel, each of the pixel signals includinginformation associated with the absorption or emission of radiation inone of the respective wavelength regions by each of the first and secondmaterials, and (ii) determine the presence of the first and secondmaterials based on the produced pixel signals.
 2. The device of claim 1,wherein the first wavelength region includes radiation wavelengthsbetween 3.15 and 3.5 microns, and the second wavelength region includesradiation wavelengths between 4.3 and 4.6 microns.
 3. The device ofclaim 1, further comprising: (e) a radiation directing arrangement fordirecting radiation from a field of view of the scene through the imageforming optical component onto the detector, such that the radiationfrom the same scene is separately imaged onto the first and secondregions of the detector surface.
 4. The device of claim 3, wherein theradiation directing arrangement includes a reflective surface deployedsubstantially parallel to the optical axis of the image forming opticalcomponent.
 5. The device of claim 3, wherein the first filter isdeployed proximate to the first region of the detector surface and thesecond filter is deployed proximate to the second region of the detectorsurface.
 6. The device of claim 3, wherein the first filter includes afirst plate deployed between the first region of the detector surfaceand the image forming optical component, and the second filter includesa second plate deployed between the second region of the detectorsurface and the image forming optical component.
 7. The device of claim3, wherein the first and second wavelength regions are in the mid waveinfrared region of the electromagnetic spectrum, and the detector issensitive to radiation in the first and second wavelength regions. 8.The device of claim 3, wherein the radiation directing arrangementincludes a first substantially wedge-shaped component and a secondsubstantially wedge-shaped component.
 9. The device of claim 8, whereinthe first filter is disposed on one of a first surface or a secondsurface of the first wedge-shaped component, and the second filter isdisposed on one of a first surface or a second surface of the secondwedge-shaped component.
 10. The device of claim 9, wherein the firstsurface of the first wedge-shaped component is a closest surface of thefirst wedge-shaped component to the image forming optical component, andthe first surface of the second wedge-shaped component is a closestsurface of the second wedge-shaped component to the image formingoptical component, and the second surface of the first wedge-shapedcomponent is a closest surface of the first wedge-shaped component tothe scene, and the second surface of the second wedge-shaped componentis a closest surface of the second wedge-shaped component to the scene.11. The device of claim 1, wherein indication of the presence or absenceof the first and second materials is based on the difference between thepixel signals produced from the first and second subsets of pixels ofthe detector.
 12. The device of claim 1, wherein the first material is ahydrocarbon gas cloud and the second material is a flame.
 13. The deviceof claim 12, wherein the electronic circuitry is further configured to:(iii) provide a measurement of the path concentration distribution ofthe hydrocarbon gas cloud based on at least a portion of the pixelsignals.
 14. The device of claim 1, wherein the first region of thedetector surface corresponds to a first half of the detector surface,and wherein the second region of the detector surface corresponds to asecond half of the detector surface.
 15. A device for imaging radiationfrom a scene the radiation including at least a separate first andsecond wavelength region, the scene including at least one of a firstand second material, the first material having spectral characteristicsin the first wavelength region and the second material having spectralcharacteristics in the second wavelength region, the device comprising:(a) a detector of the radiation from the scene, the detector including afirst detector region and a second detector region, the first and seconddetector regions being separate; (b) a static filtering arrangementincluding a first and second filter, each of the filters having arespective pass band and a stop band, the first wavelength region beingwithin the pass band of the first filter and the stop band of the secondfilter, and the second wavelength region being within the pass band ofthe second filter and the stop band of the first filter, the firstfilter filtering a first portion of radiation from the field of view ofthe scene, and the second filter filtering a second portion of radiationfrom the field of view of the scene; (c) an image forming opticalcomponent for forming an image of the scene on the detector, a field ofview of the scene being defined in part by the image forming opticalcomponent; and (d) a reflective surface deployed substantially parallelto the optical axis of the image forming optical component, thereflective surface reflecting the first portion of radiation from thefield of view of the scene through the image forming optical componentonto the first detector region, such that, the radiation from the samescene is imaged separately and simultaneously onto the first and seconddetector regions and the imaged radiation on the first detector regionincludes radiation in the first wavelength region and the imagedradiation on the second detector region includes radiation in the secondwavelength region.
 16. The device of claim 15, wherein each of the firstand second filters is fixedly positioned at a distance from the imageforming optical component greater than a minimum threshold distance, theminimum threshold distance being determined according to a ratio betweenthe aperture diameter of the image forming optical component and afunction of half of the field of view defined by the image formingoptical component.
 17. The device of claim 15, wherein the image formedon the first detector region is inverted relative to the image formed onthe second detector region.
 18. The device of claim 15, wherein thedetector has a detector surface, and the detector includes: (i) a firstregion of the detector surface including a first subset of detectorpixels, and (ii) a second region of the detector surface including asecond subset of detector pixels.
 19. The device of claim 18, furthercomprising: (e) electronic circuitry electronically coupled to thedetector, the electronic circuitry configured to: (i) produce a pixelsignal from each respective detector pixel, each of the pixel signalsincluding information associated with the absorption or emission ofradiation in one of the respective wavelength regions by each of thefirst and second materials, and (ii) determine the presence of the firstand second materials based on the produced pixel signals.